With the advancement of electronic technology, particles have been put to wide, practical use in various fields. Among such particles, there maybe mentioned particles used as spacers in liquid crystal display devices, for instance.
In one of the fields of application of such particles, liquid crystal display devices, for instance, are widely used in personal computers, portable electronic apparatus and the like. Generally, a liquid crystal display device comprises, as shown in FIG. 75, a liquid crystal layer 7 sandwiched between two paired insulating substrates 1, on which color filters 4, a black matrix 5, transparent electrodes 3, an alignment layer 9 and so on are formed.
The distance between the above paired insulating substrates 1, namely the thickness of the liquid crystal layer, influences the transmittance of light and, therefore, if the liquid crystal layer thickness is not maintained constant all over the display area of a liquid crystal display device, satisfactory display will not be attained. For this reason, spacers 8, for example glass fibers or truly spherical plastic beads, are disposed between the paired insulating substrates so that the liquid crystal layer thickness may be maintained constant all over the display area.
These spacers are dispersed uniformly on the alignment layer, for example, by spraying, together with a compressed gas, from a nozzle (dry spraying) or spraying of a liquid composed of spacers and a volatile liquid (wet spraying) after alignment layer formation. Thereafter, the insulating substrate is paired with a counterpart insulating substrate for panel alignment and a liquid crystal, for example a nematic liquid crystal, is filled into the space between the paired insulating substrates with spacers sandwiched therebetween.
When, however, spacers are disposed also on pixel electrodes within the display area, light leakage occurs from such spacers and the substantial aperture ratio is thereby reduced, so that such problems as display unevenness and reduced contrast arise.
For solving such problems as mentioned above, it is only necessary to dispose spacers only in those electrode gaps which are nondisplay areas, namely only at sites of a black matrix, which is constituted of a light shield layer. The black matrix is provided for the purpose of improving the display contrast of the liquid crystal display device and, in the case of TFT type liquid crystal display devices, for the purpose of preventing their elements from erroneously operating in response to external light.
For TFT type liquid crystal display devices, a technology of disposing spacers at sites corresponding to the black matrix, namely at sites other than display pixel sites, is disclosed in Japanese Kokai Publication Hei-04-256925 which comprises maintaining the gate electrode and drain electrode at the same electric potential in the step of spraying spacers. Further, Japanese Kokai Publication Hei-05-61052 discloses a method comprising applying a positive voltage to the circuit electrodes and charging spacers negatively and spraying them by dry method. In these technologies, it is intended to control spacer disposition by applying a voltage to electrodes formed on the substrate.
However, they have a problem. Namely, application of a voltage to the substrate having thin film transistors (TFTs) formed thereon, for the purpose of controlling the spacer disposition, may lead to destruction of elements by that voltage, hence to failure to function as a liquid crystal display device.
There is another problem. Namely, such technologies as mentioned above cannot be employed in STN (supertwisted nematic) type liquid display devices since the sites corresponding to the black matrix are spaces among transparent electrodes.
On the other hand, as a technology of disposing spacers in spaces between stripe-form transparent electrodes constituted by disposing a plurality of linear transparent electrodes in parallel on a substrate, as in STN type liquid crystal display devices, there are disclosed, in Japanese Kokai Publication Hei-03-293328 and Japanese Kokai Publication Hei-04-204417, methods of producing liquid crystal display devices which comprise charging spacers either positively or negatively and applying a voltage of the same polarity to the transparent electrodes on the substrate in the step of spacer spraying.
In particular, according to Japanese Kokai Publication Hei-04-204417, a conductor is disposed below the electrode substrate in a spacer sprayer for positive voltage application so that the velocity of falling of negatively charged spacers may be controlled. It is further disclosed that, for avoiding adhesion of negatively charged spacer particles to the wall of the spray chamber, the chamber should be made of a conductor to enable negative voltage application.
However, when, in practicing these methods, the spacer charge amount and/or the voltage to be applied to electrodes is selected at a low level (voltage value: not higher than about 1,000 V), the repulsive force (repellent force) between spacers and electrodes becomes weak, and the force for shifting spacers to interelectrode spaces becomes insufficient, hence the selectivity toward spacer disposition in electrode-free areas (interelectrode areas) becomes poor, with the result that a number of spacers are disposed also on each electrode, as shown in FIG. 76.
Conversely when the spacer charge amount and/or the voltage to be applied to electrodes is increased (voltage value: about several kilovolts), the repulsive force between spacers and electrodes becomes strong and the selectivity toward spacer disposition in electrode-free areas (interelectrode areas) is improved, as shown in FIG. 77.
In this case, however, the repulsive force acts more strongly over the set of electrodes, so that the tendency of spacers to be turned out of the domain of the electrodes increases; as a result, no spacers are disposed at all in the peripheral region of the electrode domain, hence the cell thickness cannot be controlled in the peripheral region of the electrode domain. Although such phenomenon occurs already at a state at which the repulsive force is still weak, the area of spacer-free portions unfavorably and markedly increases as the repulsive force increases.
In Japanese Kokai Publication Hei-08-76132, there is disclosed a method of disposing spacers more selectively as compared with the methods mentioned above. The method comprises charging spacers to be sprayed either positively or negatively, applying a voltage opposite in polarity of the spacer charge to first electrodes provided in areas on the insulating substrate where spacers are to be disposed, and applying a voltage of the same polarity as the spacer charge polarity to second electrodes provided in areas on the insulating substrate where no spacers are to be disposed, to thereby apply a repulsive force and an attractive force between spacers and the electrodes so that the spacers may be disposed either on the first electrodes or on the second electrodes with good selectivity.
This method, however, has a problem in that the contrast is decreased by the occurrence of spacers on the electrodes. Another problem is that when this method is applied to the production of simple matrix type liquid crystal display devices, it is necessary to form electrodes for spacer disposition in addition to the pixel electrodes and the aperture ratio decreases accordingly.